1. Field of the Invention
The present invention relates to synthetic quartz glass blanks having a good transmittance and low deterioration during laser irradiation from which can be obtained optical elements such as lenses, prisms, mirrors and windows for use with excimer lasers, and particularly ArF excimer lasers.
2. Prior Art
Higher levels of integration in VLSI circuits have led to exposure patterns of increasingly small linewidth. This has created a need for exposure light sources of shorter wavelength in the lithography systems used to form circuit patterns on semiconductor wafers. The i-line (wavelength, 365 nm), once the light source of choice in lithography steppers, has been largely supplanted by the KrF excimer laser (248 nm), and today ArF excimer lasers (193 nm) are starting to see industrial use.
This trend toward shorter wavelength light sources has also created a need for higher precision in the optical components (e.g., lenses, windows, prisms) used in exposure tools. Some of the many important concerns that exist regarding such components, particularly when used with ArF excimer lasers, include refractive index homogeneity, improving the transmittance and reducing the scattering of laser light, and stability to excimer laser irradiation.
Of these concerns, the refractive index homogeneity xcex94n is the most critical and most difficult to achieve. The hydroxyl group concentration and its distribution have a large influence on the refractive index distribution in quartz glass. That is, a hydroxyl group concentration of 10 ppm reportedly narrows the refractive index distribution in quartz glass by 1xc3x9710xe2x88x926. It can readily be seen from this that a very high-homogeneity synthetic quartz glass body having a hydroxyl group concentration distribution of only 10 ppm would be needed to obtain a synthetic quartz glass blank in which xcex94n=1xc3x9710xe2x88x926.
Two methods are commonly used for making synthetic quartz glass: a direct method in which a silica-forming starting material is flame hydrolyzed, forming fine particles of silica which are then melted and deposited to effect growth; and a soot method in which a silica-forming starting material is flame hydrolyzed, forming fine particles of silica which are deposited to effect growth, then later vitrified to form a clear glass. However, obtaining a synthetic quartz glass body of such high homogeneity directly by either of these methods is technically very difficult. To obtain a synthetic quartz glass body of higher homogeneity, it is thus necessary to subject the synthetic glass ingot obtained by either method to homogenizing treatment.
The most efficient and effective way to homogenize quartz glass is to carry out the zone melting process disclosed in JP-A 7-267662 in the ingot growth direction and in a direction perpendicular thereto. This approach has a number of advantages. For example, the molten portion of the ingot is mechanically agitated, enabling efficient homogenization to be carried out and thus making it possible to narrow, for example, the distribution in the hydroxyl group concentration. In addition, during homogenization, the quartz glass ingot is treated without being brought into contact with anything other than the burner flame, minimizing the diffusion of external impurities to the ingot and thus holding down the decline in UV light transmittance.
Generally, when homogenizing treatment by a zone melting process is used to improve the uniformity of the hydroxyl group concentration, a wider variation in hydroxyl group concentration prior to such treatment results in less efficient homogenization. The efficiency of homogenization declines also with increasing hydroxyl group concentration. At higher hydroxyl group concentrations in particular, the variation in concentration is generally wider, detracting even further from the efficiency of homogenization. For this reason, a hydroxyl group concentration of 1,000 ppm or less is preferred in synthetic quartz glass ingots subjected to homogenization.
Other properties which, like the refractive index homogeneity xcex94n, are of critical importance in synthetic quartz glass blanks for optical elements used in ArF excimer laser exposure systems, are the transmittance of the glass to UV light and its stability to laser irradiation.
The most important transmittance to UV light is the transmittance to the 193 nm wavelength light used in an ArF excimer laser. The transmittance of quartz glass to light at this wavelength decreases as the content of impurities rises. Typical impurities include alkali metals such as sodium, and other metallic elements such as copper and iron. By using a silane or silicone starting material of very high purity to produce the synthetic quart glass, the concentration of such metallic impurities present within the quartz glass can be brought down to below the level of detection by a highly sensitive detector ( less than 1 ppb). However, because sodium and copper have relatively large coefficients of diffusion to quartz glass, the diffusion and admixture of such external impurities often occurs during homogenization and heat treatment. Special care must be taken to avoid such contamination during these treatment operations.
Stability of the quartz glass to excimer laser irradiation is a very important factor, particularly as an ArF excimer laser reportedly causes five times more damage than a KrF excimer laser.
When quartz glass is irradiated with ArF excimer laser light, one effect that arises is the cleavage of Sixe2x80x94Oxe2x80x94Si bonds by the very intense energy of the light, forming the paramagnetic defects commonly known as Exe2x80x2 centers which absorb 215 nm light. Another effect, commonly referred to as xe2x80x9claser compaction,xe2x80x9d is a rearrangement of the network structure of quartz glass that increases the density of the glass.
The former effect lowers the transmittance of the quartz glass to 193 nm light, and the latter effect raises the refractive index and increases the birefringence. All of these changes in optical characteristics are undesirable for an exposure system.
It is known that reducing the number of intrinsic defects in quartz glass and setting the hydrogen concentration in the glass to at least a certain level are both highly effective for improving the stability of the quartz glass to laser irradiation.
Intrinsic defects present in quartz glass include defects characterized by too much or too little oxygen for the Sixe2x80x94Oxe2x80x94Si structure making up the quartz glass. Well-known examples include oxygen deficient defects (Sixe2x80x94Si, which absorbs at 245 nm) and oxygen surplus defects (Sixe2x80x94Oxe2x80x94Oxe2x80x94Si, which absorbs at 177 nm). However, such defects, or at least those which are measurable by spectrophotometric means, are excluded from optical-grade synthetic quartz glass to begin with. Of greater concern are more subtle defects, such as those in which the Sixe2x80x94Oxe2x80x94Si bond angle falls outside the range of stability, as in the case of excessively stretched or compressed Sixe2x80x94Oxe2x80x94Si bonds.
To remove such unstable structures, JP-A 7-61823 discloses a process in which the growth rate of quartz glass produced by the direct method is held to a level of not more than 2 mm per hour.
Although this process does appear to work, because the growth rate is very slow, it has a poor productivity and is not very cost-effective. Moreover, with regard to the general production conditions, it is empirically known that a slow growth rate tends to increase the hydroxyl group concentration in the resulting quartz glass. Two examples are cited in JP-A 7-61823, but the synthetic quartz glass obtained in both had hydroxyl group concentrations of 1,200 ppm, which is considerably higher than 1,000 ppm.
Because, as noted above, hydroxyl groups have a large impact on the refractive index of quartz glass, a lower hydroxyl group concentration is preferred for obtaining a more uniform refractive index distribution. Homogenization of the resulting quartz glass body is not called for in the art disclosed in JP-A 7-61823. However, in cases where homogenizing treatment is subsequently carried out to increase the uniformity of the refractive index, it is preferable for the hydroxyl group concentration to be no higher than 1,000 ppm. At a concentration above 1,000 ppm, the efficiency of homogenization declines, lengthening the length of time required for treatment. A longer treatment time increases the diffusion of external impurities into the quartz glass, thus lowering the transmittance of the glass, and also reduces the hydrogen concentration.
The fact that hydrogen molecules in the quartz glass inhibit damage to the glass by excimer laser irradiation is well-known in the art and has been the subject of active investigation ever since it was revealed in JP-A 1-212247.
There are two ways to include an appropriate level of hydrogen molecules in quartz glass. One method is to introduce hydrogen molecules into the growing ingot by suitably adjusting the ratio of hydrogen, propane and oxygen used as the combustion gases during growth of the quartz glass ingot. This approach allows the amount of hydrogen molecules that dissolve in the growing ingot to be adjusted within a range of about 0 to 2xc3x971019 molecules/cm3.
The other method involves the thermal diffusion of hydrogen molecules by heat treating a quartz glass body within a hydrogen atmosphere. This method has the advantage of enabling strict control of the hydrogen molecule concentration. At the same time, it also has a number of significant disadvantages. Specifically, because it uses hydrogen gas, which is a highly flammable substance, there is a risk of explosion. Also, the associated equipment costs for safety and other purposes represent a substantial economic burden. In addition, heat treatment as in this case may allow impurities to diffuse into the quartz glass, which tends to lower the transmittance of the glass.
An important factor which affects the refractive index distribution of the quartz glass and governs the stability of the glass to ArF excimer laser irradiation is the xe2x80x9cfictive temperature.xe2x80x9d This is a concept particular to glass, and refers to the temperature at which glass in a molten state, as it cools, undergoes a loss in the freedom of the molecules and solidifies. The physical value to which the fictive temperature relates is the density of the quartz glass.
Because the fictive temperature distribution of quartz glass is also, like the hydroxyl group concentration distribution, a major determinant of the refractive index distribution in the quartz glass, methods for holding the refractive index distribution xcex94n in a quartz glass body to 1xc3x9710xe2x88x926 or less by suitably combining the hydroxyl group concentration distribution and the fictive temperature distribution have been disclosed in JP-A 2-102139 and JP-A 2-239127.
We have found from our own research that the fictive temperature has a large influence on changes in the transmittance of quartz glass when irradiated with ArF excimer laser light. It thus became clear to us that the intrinsic defects present in quartz glass, its hydrogen concentration, and its fictive temperature are all important factors for enhancing the laser durability of the quartz glass. This will be discussed more fully later in the specification.
JP-A 2-102139 and JP-A 2-239127 devote attention to planarizing the refractive index distribution, and thus cite suitable ranges for the fictive temperature distribution. Yet, because nothing was known at the time about how the fictive temperature interacts with laser durability, the fictive temperature itself is not discussed. Hence, these prior-art disclosures do not resolve the technical issue addressed by the present invention; namely, how to improve both the refractive index distribution and the laser durability of synthetic quartz glass blanks.
JP-A 5-58667 teaches that the resistance of quartz glass to damage from excimer laser irradiation is enhanced by controlling the fictive temperature of the glass within a range of 800 to 1,000xc2x0 C. However, as is apparent from the description given therein, JP-A 5-58667 is concerned with KrF excimer laser and not ArF excimer lasers. Hence, the fictive temperature range is broader. Quartz glass material intended for use in the production of optical elements for ArF excimer laser exposure systems must have an optimized fictive temperature.
It is therefore an object of the present invention to provide synthetic quartz glass blanks which have a good transmittance to laser light and minimal deterioration during laser irradiation, and are thus suitable for excimer laser-related applications, particularly ArF excimer laser-related applications.
We have discovered that synthetic quartz glass blanks endowed with characteristics (a) to (d) listed below provide optical elements which have a good transmittance and experience minimal deterioration, and can thus be used in excimer laser applications, particularly ArF excimer laser applications.
Accordingly, the invention provides a synthetic quartz glass blank which is obtained by homogenizing a synthetic quartz glass ingot having periodic striae in a direction of growth, has a generally cylindrical shape with a diameter of 150 to 380 mm and a thickness of 50 to 150 mm, and contains substantially no chlorine; wherein the blank has:
(a) striae grades in a working direction and an off-axis direction which meet grade A of U.S. military specification MIL-G-174B,
(b) a working direction hydroxyl group concentration averaged in the off-axis direction and an off-axis direction hydroxyl group concentration averaged in the working direction of 700 to 1,000 ppm each,
(c) a working direction fictive temperature averaged in the off-axis direction and an off-axis direction fictive temperature averaged in the working direction of 850 to 950xc2x0 C. each, and
(d) a refractive index distribution xcex94n for 633 nm wavelength light in the working direction of the synthetic quartz glass of at most 1xc3x9710xe2x88x926.
Preferably, the synthetic quartz glass blank of the invention, after irradiation with 30,000 pulses of ArF excimer laser light at an energy density per pulse of 2 mJ/cm2 and a frequency of 200 Hz, has a laser light transmittance that is at least 98.0% of the transmittance prior to laser irradiation and, after irradiation with 2xc3x97106 pulses of ArF excimer laser light under the same conditions, has a transmittance of at least 97.5%.
The inventive synthetic quartz glass blank typically has an average hydrogen molecule concentration in the working direction of 2xc3x971017 to 3xc3x971018 molecules/cm3.
It is advantageous for the striae on the ingot from which the synthetic quartz glass blank of the invention is obtained to be distributed periodically in the growth direction to a density of at least one striae per centimeter and to be located preferably at positions where feeding of a silica-forming starting compound is interrupted during growth of the ingot.
The ingot from which the inventive blank is obtained is generally repeatedly homogenized in the growth direction and in a direction perpendicular to the growth direction.